Field of the Invention
The present invention concerns a process for manufacturing a thermoformable plasticized composite containing cellulose fiber and polylactic acid (PLA), as described in the preamble of claim 1, which composite has an improved impact strength.
Description of Related Art
Natural fibre and cellulose based biocomposite and natural fibre composite (NFC) materials, as well as their use in replacing plastic products, have been under intensive study for over a decade due to several reasons, e.g. the renewable character of these raw materials, the low weight of the composites, the low price of fibres compared to plastics, the new product properties compared to plastics, the end product image and the new product possibilities for these cellulose materials.
When looking for new product possibilities for cellulose fibre, one natural way is to go towards a mouldable fibre web or towards composite materials with high fibre contents, which materials are thermally mouldable. Those materials allow the direction towards plastic-type products, such as packaging materials, having high mouldability to shapes and forms completely new for materials with a high cellulose content.
One challenge in combining highly hydrophilic cellulose and hydrophobic polymeric materials is a poor adhesion, which restrict film forming properties and gives low impact strength for these materials. The use of cellulose in biocomposites requires some kind of plasticization. However, the use of typical plasticizers (e.g. non-bonded plasticizer systems) often deteriorates temperature resistance, impact resistance, water absorption properties and they have a tendency to migrate on the material surface.
Reactions to cellulose are normally done using different solvent processing methods, those are often time, energy and solvent consuming.
One of the most critical properties of polylactide-based biocomposites is their impact strength. However, for example composites of polylactide and cellulose fibers have typically been quite fragile, with low impact strengths.
In most prior solutions for forming thermoformable plasticized fiber products, the fiber is modified or mixed into a water/solvent suspension, whereby no reactive components, such as anhydrides, can be attached to the surface of the product without losing its reactivity.
On the other hand, these prior products have had extremely low fiber contents (about 10%).
Thus, there is a need for methods that make it possible to manufacture products with high fiber contents, particularly high contents of cellulose fiber, which products exhibit good mouldability and high impact strengths, thus making them suitable for use in replacing plastic products.